FIG. 1 illustrates a conventional LED die 12 having a layer of phosphor 14 overlying and separated from the LED die 12, but proximate to the LED die 12. The LED die 12 includes n-type layers 15, an active layer 16 that emits light, and p-type layers 17. Light exits through the top and side surfaces of the LED die 12. The LED die 12 may emit blue light, and the phosphor 14 may be a YAG phosphor that emits a yellow-green light. The combination of the blue light leaking through the phosphor 14 and the phosphor emission creates white light. The phosphor 14 may be supported by a transparent support surface in close proximity to the LED die 12.
FIG. 2 illustrates the generally Lambertian light emission pattern 18 of the bare LED die 12. When the light impinges on a flat layer of phosphor 14 above the LED die 12, the flux impinging on the phosphor 14 is greatest along the normal axis directly over the center of the LED die 12. The flux drops off with distance from the center point over the LED die 12. In other words, as the angle of the light rays deviates from normal, the flux impinging on the phosphor 14 becomes less. As a result, the light exiting the surface of the phosphor 14 will be bluer directly over the LED die 12 relative to the light further away from the center of the LED die 12.
Additionally, for the same reasons, the temperature of the phosphor 14 will be higher directly over the LED die 12 due to the higher flux. Some phosphors and quantum dots are very sensitive to temperature and change their optical characteristics with temperature. Therefore, the temperature gradient across the phosphor 14 will contribute to the color non-uniformity. Also, at higher temperature locations, the quantum efficiency of the phosphor is reduced as well as the long term material stability.
Additionally, the phosphor directly over the LED die 12 suffers other adverse effects from the high flux, which affects the color directly over the LED die 12.
Accordingly, the color will not be uniform across the surface of the phosphor 14. In some applications, such as when the image of the phosphor-converted LED die 12 is magnified by a parabolic mirror in a projector, headlight, or other secondary optics for a spot application, the color non-uniformity is particularly noticeable.
These problems also exist in LED modules where an array of LED dies is mounted on a common substrate and covered with a layer of phosphor. Above each LED die is an optical and thermal hot spot, where the light is bluer (assuming blue LED dies are used) and where the phosphor is adversely affected by temperature and other factors. As a result, the color and temperature is not uniform across the phosphor.
What is needed is a technique for improving the color uniformity of light across a phosphor surface excited by light from one or more LED dies.